An exhaust gas sensor (e.g., exhaust oxygen sensor) may be positioned in an exhaust system of a vehicle and operated to provide indications of various exhaust gas constituents. In one example, the exhaust gas sensor may be used to detect an air-fuel ratio of exhaust gas exhausted from an internal combustion engine of the vehicle. The exhaust gas sensor readings may then be used to control operation of the internal combustion engine to propel the vehicle. Additionally, a first estimate of an alcohol content of fuel burned in the engine may be determined based on the air-fuel ratio. For example, U.S. Pat. No. 6,016,796 describes a method for determining an air-fuel ratio following a re-fueling event and then updating a fuel ethanol content estimate based on the determined air-fuel ratio.
In another example, outputs of the exhaust gas sensor may be used to estimate a water content in the exhaust gas. Water content estimated using the exhaust gas oxygen sensor may be used to infer an ambient humidity during engine operation. Further still, the water content may be used to infer a second fuel ethanol content estimate. Under select conditions, the exhaust gas sensor may be operated as a variable voltage (VVs) oxygen sensor in order to more accurately determine exhaust water content and fuel ethanol content. When operating in the VVs mode, a reference voltage of the exhaust gas sensor is increased from a lower, base voltage (e.g., approximately 450 mv) to a higher, target voltage (e.g., in a range of 900-1100 mV). In some examples, the higher, target voltage may be a voltage at which water molecules are partially or fully dissociated at the oxygen sensor while the base voltage is a voltage at which water molecules are not dissociated at the sensor.
However, the inventors herein have recognized that each of the above-described methods for estimating the fuel ethanol content may have various noise factors (e.g., ambient humidity, pressure, air-fuel ratio) that may reduce the accuracy of the estimate under certain operating conditions. Further, operation of the exhaust oxygen sensor in the VVs mode may not be possible until engine temperatures have increased above a threshold level. Further still, continuously operating the exhaust oxygen sensor in the VVs mode, and particularly at the higher target voltage, may result in sensor degradation. Inaccurate fuel ethanol content estimates may result in reduced engine control.
In one example, the issues described above may be addressed by a method for estimating a first fuel alcohol content based on an air-fuel ratio estimated with an exhaust oxygen sensor; after an engine temperature increases above a threshold, estimating a second fuel alcohol content based on a change in sensor output during modulating a reference voltage of the exhaust oxygen sensor between a first and second voltage; and adjusting engine operation based on a difference between the first and second fuel alcohol contents. In this way, errors in the fuel alcohol content estimate may be reduced and a more accurate fuel alcohol content estimated may be selected for engine control, thereby increasing engine performance and fuel economy.
As one example, responsive to modulating the voltage of the exhaust oxygen sensor between the first and second voltages, first and second pumping currents may be generated. The first pumping current may be indicative of an amount of oxygen in a sample gas while the second pumping current may be indicative of the amount of oxygen in the sample gas plus an amount of oxygen contained in water molecules in the sample gas. The first and second pumping currents may then be corrected based on one or more of deviations of an expected air-fuel ratio (at which the engine is thought to be operating) from an estimated air-fuel ratio (at which the engine is actually operating), ambient humidity, pressure, and a water vapor environment of the sensor (e.g., whether the engine is current injecting fuel or not). The corrected values may then be used to compute a water content, and infer an alcohol content of burned fuel with higher accuracy and reliability. However, since operating the oxygen sensor at the higher second voltage may degrade the sensor time, it may be desirable to adjust engine operation based on the first fuel alcohol content determined while the oxygen sensor is operating at the first voltage. For example, when the difference between the first and second fuel alcohol content estimates is less than a threshold, an engine controller may adjust engine operation based on the first fuel alcohol content and not the second. Conversely if the difference between the first and second fuel alcohol content estimates is greater than the threshold, the engine controller may adjust engine operation based on the second fuel alcohol content and not the first. In this way, following an engine re-fueling event, a fuel alcohol content estimate may be determined. By comparing the two different estimates, the most accurate fuel alcohol content estimate may be selected and used for increased engine control while at the same time reducing the amount of time the sensor spends operating in a variable voltage mode.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.